2.1. Heparan Sulfate and Related Compounds
The plasma membrane, extracellular matrix, and basement membranes of all tissue types contain, among other constituents, complex macromolecules referred to as heparan sulfate proteoglycans (HSPG). Their function in biological processes is believed to be varied: they appear to be involved in cell-cell recognition, tissue differentiation and morphogenesis, organization of extracellular matrix, permaselective properties, and cell-substrate adhesion. The extracellular matrix appears to be essential to the control of cell proliferation and morphogenesis, and HSPG, as a principal component of basement membranes, plays an integral role in tissue architecture and function.
Proteoglycans are high molecular weight compounds with a protein backbone; linked to the backbone are a number of side chains of different types of heteropolysaccharides. A high proportion of the molecular weight may thus be attributed to carbohydrates. These compounds can routinely be broken down by a number of different enzymes. The degradation of the proteoglycans usually begins with the proteolytic cleavage of the backbone to produce peptide components and glycosaminoglycans. The latter are in turn hydrolysable into smaller glycosaminoglycan fragments by endoglycosidase enzymes, and these are further degradable into monosaccharides by exoglycosidases. Heparan sulfate proteoglycans have an intimate interrelationship with the other macromolecules which make up the extracellular matrix, and thus, their degradation may have a profound effect on the regulation of cell anchorage, movement, function and growth.